The present invention relates generally to a method for measuring nuclear magnetic resonance properties of an earth formation traversed by a borehole, and, more particularly, to a method for reducing ringing artifacts in a nuclear magnetic resonance measurement.
Nuclear magnetic resonance (NMR) measurements taken in a borehole can provide different types of information about a geological formation. In the past, such measurements often were made after the borehole had been drilled. Today, however, it is possible to log NMR measurements while drilling (i.e., logging while drilling or LWD), thus saving time and providing valuable real-time information about the earth formation as drilling progresses. For example, such information can indicate the fractional volume of pore space, the fractional volume of mobile fluid, the total porosity of the formation, etc.
NMR logging tools generally include one or more large permanent magnets or electromagnets for generating a static magnetic field, B0, an antenna placed proximate the formation to be analyzed, and circuitry adapted to conduct a sequence of RF power pulses through the antenna to induce an RF magnetic field, B1, in the formation. The circuitry also includes a receiver adapted to detect signals induced in the antenna as a result of the RF pulse sequence. The induced signals then can be measured and processed to provide the desired information about the properties of the formation.
Typically, NMR logging tools are tuned to detect hydrogen resonance signals (e.g., from either water or hydrocarbons) because hydrogen nuclei are the most abundant and easily detectable. In general, measurements of NMR related phenomena of hydrogen nuclei in the earth formation are performed by allowing some time for the static magnetic field, B0, to polarize the hydrogen nuclei of water and hydrocarbons in a direction substantially parallel to B0, thus creating a nuclear magnetization. The direction of the nuclear magnetization can then be changed by applying a sequence of RF pulses to induce the RF field B1. Commonly, the pulse sequence employed includes a first RF pulse (i.e., the excitation pulse) having a magnitude and duration selected to re-orient the nuclear magnetization by about 90 degrees from the orientation attained as a result of B0 (i.e., the initial transverse magnetization). After a selected time, a train of successive RF pulses is applied (i.e., inversion or refocusing pulses), each of which has a magnitude and a direction selected to re-orient the nuclear spin axes by about 180 degrees from their immediately previous orientations. The frequency of the RF field needed to re-orient the nuclear magnetization (i.e., the Larmor frequency) is related to the amplitude of the static magnetic field B0 by the gyromagnetic ratio y, which is unique to each isotope.
After application of the initial RF pulse (i.e., after the nuclear magnetization is in the plane perpendicular to B0), the nuclear magnetization begins to precess around B0, producing a weak RF signal at the Larmor frequency which is detectable by the antenna. Due to inhomogeneities in the magnetic field B0, the coherence between the individual spins eventually is lost and the nuclear magnetization decays rapidly. The inversion pulses re-create the lost magnetization (i.e., the coherence re-appears), producing signals that can be detected by the antenna. These signals, referred to as “spin echoes,” generally are measured during the time interval between successive RF inversion pulses. The rate at which the spin echoes decay (i.e., the rate at which the nuclei irrevocably lose their alignment within the transverse plane) is referred to as the transverse relaxation rate. The time constant of this decay, referred to as the traverse relaxation time T2, is related to the chemical and physical properties of the earth formation. For example, hydrogen nuclei in viscous oils have relatively short relaxation times, whereas hydrogen nuclei in light oils have relatively long relaxation times. Similarly, hydrogen nuclei in free water typically have longer relaxation times than those in bound water (e.g., clay-bound water).
To acquire the NMR data, several known pulse sequences are commonly employed. Such sequences include Carr-Purcell-like sequences, such as the Carr-Purcell-Meiboom-Gill (“CPMG”) pulse sequence, which often is used for measuring transverse relaxation times. Advantages of the CPMG pulse sequence include compensation for pulse imperfections and inhomogeneities in both the B0 and B1 magnetic fields, as is well known in the art.
The Carr-Purcell-like sequences, however, do not compensate for all undesirable artifacts appearing in NMR measurements. For example, exciting the antenna with RF power pulses in the presence of a strong static magnetic field causes mechanical excitation of the antenna, which leads to generation of a spurious signal in the antenna called “ringing.” The ringing signal is unrelated to the NMR phenomena and typically has a large amplitude and slow rate of decay relative to the induced spin echo signals. Because the spin echoes typically are measured shortly after application of an RF pulse, the ringing signals often overwhelm the spin echo signals, resulting in loss of significant information regarding the formation properties.
Several different techniques are known for reducing ringing. One technique has been to design the hardware to minimize the interaction between the electromagnetic fields and the materials of the hardware. An example of such hardware is described in U.S. Pat. No. 5,712,566 issued to Taicher et al.
Another technique, known as “phase alternating pairs” or PAPs, includes creating a phase difference between the ringing signal and the spin echo signals, and summing or “stacking” multiple echo sequences to reduce the amplitude of the ringing signal in the final output. For example, to compensate the ringing of the 180 degree pulses in a particular pulse sequence, the sequence may be repeated twice with opposing directions of the initial 90 degree pulse. By stacking the two sets of echo measurements, the ringing contribution of the 180 degree pulses can be substantially canceled. The ringing of the 90 degree pulse, on the other hand, is not canceled. However, because the 90 degree pulse is only applied once at the beginning of each sequence, its influence dies down relatively quickly and thus affects only early spin echo signals. Examples of ringing cancellation methods utilizing PAPs are described in U.S. Pat. No. 5,596,274 issued to Sezginer and U.S. Pat. No. 5,023,551 issued to Kleinberg et al., and International Publication No. WO 98/43064 by Numar Corp.
A necessary assumption underlying the phase alternated pair ringing cancellation technique is that the ringing does not change between the two measurement sequences. However, in borehole logging applications, the logging tool is moving continuously as the NMR measurements are being made. It has been found in such applications that movement and, in particular, bending of the tool changes the characteristic of the ringing signal between consecutive measurements. Because the two measurements forming a phase alternating pair are separated by a wait time of typically a few seconds, the orientation of the tool in the borehole during a first sequence bears no relationship to the orientation of the tool during the successive sequence. Thus, the phase alternating pair cancellation technique may not result in complete cancellation of the ringing signal in such applications.
Accordingly, it would be desirable to provide a new pulse sequence and a method of processing the spin echo signals resulting from the new pulse sequence to cancel the ringing.